A direct current (DC)-DC converter is typically used to step down a power supply voltage to meet the needs of a particular circuit. In many instances, such circuits have variable loads—for example, in DC motors, which are found in innumerable types of electronic products, from medical equipment to automobiles. To conserve power while maintaining the proper voltage supply to the variable load, DC-DC converters often employ pulse width modulation (PWM), in which an input voltage that is rapidly switched on and off is applied to an output filter to regulate the voltage and current supplied to the load in an efficient manner. A PWM controller is used to control this switching operation. Many such controllers (e.g., current mode controllers) monitor the current flowing through a converter output inductor and use the inductor current signal as part of a feedback loop to improve the stability of the PWM control system.
Current mode controllers have improved stability relative to other types of controllers. However, they respond slowly to changes in the load current. This slow response results in deviations in the voltage supplied to the load. It is therefore desirable to maintain the superior stability of current mode control systems while improving the response time to changes in the load. This can be accomplished with the load current feed forward control method. Available load current feed forward control methods entail the sensing of load current using a series impedance between the output filter capacitor and the load. This approach is disadvantageous, however, because the impedance can produce added power loss and interfere with the voltage regulation to the load. A simple, lossless, non-interfering method of estimating the output current is therefore desired.